Integrated Process for Urea and Melamine Production

ABSTRACT

In an integrated process for urea and melamine production, urea is produced in a urea plant ( 10 ) comprising a high pressure urea synthesis section ( 11 ) from which an aqueous solution comprising urea, ammonium carbamate and ammonia is obtained and a urea recovery section ( 21 ) operating at low pressure, and melamine is produced in a melamine plant ( 40 ) wherein off-gases resulting as by-products of the melamine synthesis are discharged from said plant at a medium pressure and recycled to the high-pressure urea synthesis section ( 11 ).

TECHNICAL FIELD

The present invention relates to a process for the integrated productionof urea and melamine.

In particular, the present invention concerns a process of theabove-identified type, wherein urea is produced in a urea plantcomprising a high pressure urea synthesis section and a urea recoverysection and wherein the off-gases resulting as by-products of themelamine synthesis are recycled to said high pressure urea synthesissection.

In the following description and subsequent claims, with the expression“high pressure urea synthesis section” it is intended to mean a sectionoperated at a pressure of at least about 120 bar, generally between130-260 bar.

More in particular, the process according to the present invention is ofthe type wherein the off-gases to be recycled have a pressure of atleast 2 bar, generally between 2 and 30 bar.

The present invention is also concerned with an integrated plant forcarrying out such a process.

As is known, in the field of urea and melamine there is increasinglyfelt the need of providing integrated processes wherein the off-gasesproduced in the melamine synthesis can be efficiently exploited for ureaproduction.

PRIOR ART

In order to meet the above requirement, integrated processes have beenproposed, wherein melamine is produced in a plant, so called melamineplant, using—as raw materials (reactants)—NH₃ and urea, the latter beingproduced in a plant for urea production, so called urea plant, to whichthe off-gases coming from the melamine plant and substantiallycontaining NH₃ and CO₂, are recycled as raw materials (reactants).

According to these processes, the off-gases, generally discharged fromthe melamine plant at a pressure comprised between 2 and 30 bar, areappropriately treated before being fed into the urea plant.

In particular, the off-gases are condensed, at a pressure equal or lowerthan their discharge pressure, with a weak ammonia aqueous solution(ammonia concentration comprised between 0 to 15% by weight). The soobtained off-gas liquid solution is then fed to a waste water treatmentsection of the urea plant, generally operated at a pressure of about 2-5bar, where NH₃ and CO₂ are recovered from the aqueous solution andrecycled to the high pressure urea synthesis section, through the lowpressure urea recovery section of the urea plant.

In the alternative, it has also been proposed to suitably compress theabove off-gas liquid solution and recycle it directly to the highpressure synthesis section of the urea plant.

Although advantageous in some extent, the above processes are affectedby several drawbacks.

In fact, in the first case, high energy consumption are required toseparate NH₃ and CO₂ from the off-gas liquid solution in the waste watertreatment section. Moreover, since the pressure in such a section isoften much lower than the pressure of the off-gases discharged from themelamine plant, the off-gases have to be expanded before theircondensation and recycle to the waste water treatment section of theurea plant, and thus there is also an energy waste in term of pressureloss.

In the second case, an additional, not negligible, amount of water isfed in the high pressure urea synthesis section through the recycledoff-gas liquid solution. Since water is a by-product of the ureasynthesis, its presence in the reactant feed is detrimental for the CO₂conversion into urea. The urea conversion yield is thus negativelyaffected by the water contained in the recycled off-gas liquid solutionwith an ensuing increase in the energy consumption required forrecovering urea from the urea solution leaving the synthesis section andfor recycling the unconverted reagents back to the synthesis section.

In addition, such processes for the integrated production of urea andmelamine are generally operated in connection with existing plants forthe synthesis of urea and melamine, respectively, and they usually allowan increase, even significant, of the urea plant capacity compared withthe design capacity for which such a plant had been designed. In orderto face such capacity increases, the prior art provides for thesubstitution of undersized equipments, in particular the equipments ofthe high-pressure synthesis loop, with new equipments. It follows thatthe implementation of urea/melamine integrated processes of the priorart, in addition to the above mentioned drawbacks, is particularlyburdensome from the economic, functional and energy consumption pointsof view.

SUMMARY OF INVENTION

The technical problem underlying the present invention is to provide anintegrated process for urea and melamine production having functionalfeatures such as to fully overcome the drawbacks set forth with respectto the prior art and in which a higher production capacity for the ureaplant can be achieved ensuring at the same time a high conversion yieldof carbon dioxide to urea in a efficient way and with low energyconsumption.

The above problem is solved, according to the invention, by anintegrated process for urea and melamine production, wherein urea isproduced in a urea plant comprising a high pressure urea synthesissection from which a aqueous solution comprising urea, ammoniumcarbamate and ammonia is obtained, and a low-pressure urea recoverysection, and melamine is produced in a melamine plant wherein off-gasesresulting as by-products of the melamine synthesis are dischargedtherefrom at a medium pressure and recycled to said high pressure ureasynthesis section, the process being characterized in that it furthercomprises the steps of:

-   -   feeding at least a part of said aqueous solution comprising        urea, ammonium carbamate and ammonia coming from said urea        synthesis section to a medium-pressure treatment section of the        urea plant for recovering ammonium carbamate and ammonia        contained in it;    -   subjecting said part of aqueous solution comprising urea,        ammonium carbamate and ammonia to dissociation in said        medium-pressure treatment section, obtaining a urea aqueous        solution and a vapour phase comprising ammonia, carbon dioxide        and water;    -   feeding said urea aqueous solution obtained from dissociation in        said treatment section to a decomposer of a urea recovery        section operating at a predetermined low pressure,    -   subjecting said urea aqueous solution to decomposition in said        decomposer of said urea recovery section, obtaining a        concentrated urea solution and a second vapour phase comprising        ammonia, carbon dioxide and water;    -   subjecting said second vapour phase to condensation in a        condenser of said urea recovery section in fluid communication        with said decomposer, obtaining a recycle ammonium carbamate        aqueous solution;    -   feeding said off-gases coming from said melamine plant and said        recycle ammonium carbamate solution to a condensation section of        said medium-pressure treatment section of the urea plant;    -   condensing said off-gases with said recycle carbamate aqueous        solution in said condenser of the medium-pressure treatment        section, obtaining a concentrated carbamate aqueous solution;    -   recycling said carbamate aqueous solution to said high pressure        urea synthesis section.

Preferably, said medium-pressure treatment section of the urea plantoperates to pressure substantially equal or lower than that of saidoff-gases discharged from said melamine plant.

In case the pressure of the recycle carbamate aqueous solution leavingthe urea recovery section is lower than the operating pressure of theoff-gas condensation section of the medium-pressure treatment section ofthe urea plant, then the process according to the present inventionfurther comprises the step of compressing said recycle carbamate aqueoussolution to a pressure substantially corresponding to the operatingpressure of said off-gas condensation section, previous to feeding it insaid condensation section.

The process of the invention further comprises the step of compressingsaid concentrated carbamate aqueous solution coming from said off-gascondensation section to a pressure substantially corresponding to theoperating pressure of said high pressure urea synthesis section,previous to feeding (recycling) it to said urea synthesis section.

According to a preferred embodiment of the invention, said condensationsection of the medium-pressure treatment section comprises a singlecondenser and the process further comprises the steps of:

-   -   feeding said vapour phase comprising ammonia, carbon dioxide and        water, said off-gases and said recycle ammonium carbamate        solution in said single condenser of the medium-pressure        treatment section;    -   condensing said vapour phase comprising ammonia, carbon dioxide        and water as well as said off-gases with said recycle ammonium        carbamate solution in said single condenser of said        medium-pressure treatment section, obtaining a concentrated        ammonium carbamate aqueous solution; and    -   recycling said concentrated ammonium carbamate aqueous solution        to said high pressure urea synthesis section.

According to another embodiment of the invention, said condensationsection of the medium-pressure treatment section comprises a firstcondenser and a second condenser in fluid communication to each otherand the process further comprises the steps of

-   -   feeding said off-gases coming from said melamine plant and said        recycle ammonium carbamate solution to said first condenser of        the medium-pressure treatment section of the urea plant;    -   condensing said off-gases with said recycle carbamate aqueous        solution in said first condenser of the medium-pressure        treatment section, obtaining a first concentrated ammonium        carbamate aqueous solution;    -   feeding said first concentrated ammonium carbamate aqueous        solution in said second condenser of the medium-pressure        treatment section;    -   feeding said vapour phase comprising ammonia, carbon dioxide and        water obtained from dissociation of said part of the aqueous        solution comprising urea, ammonium carbamate and ammonia, in        said second condenser of the medium-pressure treatment section;    -   condensing said vapour phase comprising ammonia, carbon dioxide        and water with said first concentrated ammonium carbamate        aqueous solution in said second condenser of the medium-pressure        treatment section, obtaining a second concentrated ammonium        carbamate aqueous solution; and    -   recycling said second concentrated ammonium carbamate aqueous        solution to said high pressure urea synthesis section.

Preferably, the process according to the invention further comprises thesteps of:

-   -   feeding carbon dioxide to said condenser of said urea recovery        section;    -   subjecting said carbon dioxide and said second vapour phase to        condensation in said condenser of said urea recovery section,        obtaining a recycle ammonium carbamato aqueous solution.

In this connection, particularly advantageous results have been obtainedby feeding a carbon dioxide amount from 1 to 10 wt. % of the totality offeed carbon dioxide to said condenser of said urea recovery section.

Preferably, said part of aqueous solution comprising urea, ammoniumcarbamate and ammonia fed to said treatment section operating at mediumpressure is comprised between 10 and 50 wt. % of said aqueous solutioncomprising urea, ammonium carbamate and ammonia obtained in saidsynthesis section.

Again preferably, said medium pressure of the treatment section iscomprised between 10 and 70 bar.

Preferably, said off-gas condensation step with said recycle carbamatesolution in said condensation section of the medium-pressure treatmentsection is of the double-effect type.

Thanks to the process according to the present invention, it hassurprisingly and advantageously been found that the amount ofcondensation water (in absolute value) necessary to recycle theunreacted ammonia and the carbon dioxide in the form of ammoniumcarbamate to the synthesis section of the urea plant is substantiallylower than the amount of condensation water (in absolute value) requiredto carry out such recycling with the processes according to the priorart, in which feed carbon dioxide and feed ammonia are fed to the mediumpressure treatment section.

This is due to the fact that, with the same production capacity of theplant for urea production, the amount of ammonia and carbon dioxide tobe recycled to the synthesis section in the form of ammonium carbamateis substantially less with the process according to the presentinvention with respect to with the processes of the prior art.

It follows that there is a significant increase in the conversion yieldof the urea synthesis section, as well as of the overall yield of theH.P. Loop, to the great advantage of the efficiency and the energyconsumption of the plant intended to carry out the process according tothe present invention.

In addition, advantageously, the condensation of the off-gases isperformed by exploiting the low amount of water already contained in therecycle carbamate aqueous solution obtained in the urea recovery sectionof the urea plant and which is anyway recycled to the high pressure ureasynthesis section. Therefore, contrary to the processes of the priorart—no additional amount of water is added to the off-gasses whenrecycling them from the melamine plant to the urea plant. It followsthat, thanks to the invention, a more concentrated carbamate solution isrecycled to the high pressure urea synthesis section of the urea plantwith the consequence that the urea conversion yield is advantageouslyincreased and the energy consumption required for recovering urea andrecycling the unconverted reagents to the synthesis section areadvantageously substantially decreased.

According to a further aspect of the present invention, the presenttechnical problem is solved by an integrated plant for implementing saidprocess, wherein urea is produced in a urea plant comprising a highpressure urea synthesis section and a low pressure urea recovery sectioncomprising a decomposer and a condenser, said section being in fluidcommunication to each other, and melamine is produced in a melamineplant wherein off-gases resulting as by-products of the melaminesynthesis are discharged therefrom at a pressure of at least 2 bar andrecycled to said high pressure urea synthesis section, the plant beingcharacterized in that it further comprises:

-   -   a medium-pressure treatment section of the urea plant comprising        a dissociator and a condensation section;    -   connecting means between said melamine synthesis section and        said condensation section of the medium-pressure treatment        section for feeding said off-gases coming from said melamine        synthesis section to said condensation section of the        medium-pressure treatment section;    -   connecting means between said condenser of the urea recovery        section and said condensation section of the medium-pressure        treatment section for feeding a recycle ammonium carbamate        coming from said condenser of the urea recovery section to said        condensation section of the medium-pressure treatment section;        and    -   connecting means between said dissociator of the medium-pressure        treatment section and said decomposer of the low-pressure urea        recovery section for feeding a urea aqueous solution obtained        from dissociation in said treatment section to said decomposer        of the urea recovery section.

According to an embodiment of the invention, said condensation sectionof the medium-pressure treatment section comprises a single condenser.

According to another embodiment of the invention, said condensationsection of the medium-pressure treatment section comprises a firstcondenser and a second condenser and the integrated plant furthercomprises:

-   -   connecting means between said melamine synthesis section and        said first condenser of the medium-pressure treatment section        for feeding said off-gases coming from said melamine synthesis        section to said first condenser of the medium-pressure treatment        section;    -   connecting means between said condenser of the urea recovery        section and said first condenser of the medium-pressure        treatment section for feeding a recycle ammonium carbamate        aqueous solution coming from said condenser of the urea recovery        section to said first condenser of the medium-pressure treatment        section;    -   connecting means between said first condenser and said second        condenser of the medium-pressure treatment section for feeding a        carbamate aqueous solution coming from said first condenser to        said second condenser;    -   connecting means between said dissociator and said second        condenser of the medium-pressure treatment section for feeding a        vapour phase comprising ammonia, carbon dioxide and water from        said dissociator to said second condenser of the medium-pressure        treatment section.

Further characteristics and advantages of the process for the urea andmelamine integrated production according to the invention will resultfrom the following description of two preferred embodiments thereofgiven by way of non limiting example with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an integrated plant for urea and melamineproduction implementing the process according to an embodiment of thepresent invention;

FIG. 2 schematically shows an integrated plant for urea and melamineproduction implementing the process according to another embodiment ofthe present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, with number 1 is globally and schematicallyindicated an integrated plant for urea and melamine production accordingto the present invention. The integrated plant 1 comprises a plant 40for the production of melamine and a plant 10 for the production of urea

The melamine plant 40 of the present invention can be of the catalyticlow pressure type (up to 70 bar) or of the non-catalytic high pressuretype (above 70 bar), provided that the off-gases discharged from themelamine plant have a pressure of at least 10 bar (medium pressure). Theplant 40 comprises a low pressure or high pressure melamine synthesissection, globally indicated with number 41.

Preferably, but non exclusively, the melamine plant 40 is of thenon-catalytic high pressure type, wherein the off-gases discharged asby-products of the melamine synthesis have a pressure comprised between10 and 70 bar, preferably between 20 and 30 bar, for example 25 bar. Ofcourse, the off-gasses discharged from the melamine plant according tothe present invention can also have a much higher pressure, depending onthe pressure at which melamine is produced.

The urea plant 10 of the present invention is of the total recycle type,and comprises—according to a first aspect of the present invention—asynthesis section 11 operating at a predetermined high pressure (atleast 120 bar, generally at about 130-260 bar), a treatment section 16operating at medium pressure (preferably 10-70 bar) and a urea recoverysection 21 operating at a predetermined low pressure (about 2-10 bar),said sections 11, 16 and 21 being in fluid communication to each other.

In the integrated process according to the invention, ammonia N andcarbon dioxide C for urea production are fed into an appropriatesynthesis section 11. In the example of FIG. 1, the urea synthesissection comprises a single reactor R.

In particular, according to such an example, the ammonia N is fed to thereactor R through a condenser 12 and the carbon dioxide C is in turn fedto the reactor R through a stripper 13 and the condenser 12.

The synthesis section 11 (reactor R), the condenser 12, the stripper 13,together with a scrubber 14 (that shall be described hereafter ingreater detail), all operate substantially at the same high pressure,thus constituting the high pressure synthesis loop (H.P. Loop) of theprocess of the present invention.

In the reactor R, or rather in the synthesis section 11, the ammonia andcarbon dioxide are made to react at the aforementioned predeterminedhigh pressure (for example comprised between 130 and 170 bar) and at apredetermined high temperature (for example comprised between 160 and200° C.). From the reactor R an aqueous solution comprising urea,ammonium carbamate and ammonia is obtained.

A part of the aqueous solution comprising urea, ammonium carbamate andammonia exiting the reactor R is suitably decompressed in a per seconventional way for example by means of a valve 15 and fed to atreatment section 16 of such an aqueous solution operating at apredetermined medium pressure, for example comprised between 10 and 70bar, preferably comprised between 15 and 30 bar, and even morepreferably comprised between 18-20 bar.

For the recovery of the ammonium carbamate and of the ammonia, the partof aqueous solution comprising urea, ammonium carbamate and ammoniasuitably decompressed is fed to a medium pressure dissociator 17 at thetreatment section 16 and subjected to dissociation obtaining an ureaaqueous solution and a vapour phase comprising ammonia, carbon dioxideand water. In particular, such a part of aqueous solution comprisingurea, ammonium carbamate and ammonia is subjected in the dissociator 17to thermal dissociation.

The vapour phase comprising ammonia, carbon dioxide and water thusobtained is then fed and subjected to condensation in a medium pressurecondenser 18 of the treatment section 16 in the way that will beexplained later.

In the example of FIG. 1, the condensation unit 18 of themedium-pressure treatment section 16 comprises a single condenser 18 a.

According to a further aspect of the present invention, the integratedprocess for urea and melamine production advantageously provides for thestep of feeding the urea aqueous solution obtained by dissociation inthe medium pressure dissociator 17 of the treatment section 16 to adecomposer 22 of a urea recovery section 21 operating at a predeterminedlow pressure, for example comprised between 1.5 and 9.5 bar, preferablycomprised between 3 and 5 bar.

For this purpose, the urea aqueous solution exiting the dissociator 17is suitably decompressed in a per se conventional way for example bymeans of a valve 20.

In particular, as represented in the preferred embodiment of the processaccording to the present invention of FIG. 1, the urea aqueous solutionexiting the dissociator 17 of the treatment section 16 is directly fedto the decomposer 22 of the urea recovery section 21.

Moreover, again in accordance with the example of FIG. 1, a part of thefeed carbon dioxide C is preferably and advantageously fed to acondenser 23 of the low pressure urea recovery section 21.

For this purpose, such a part of feed carbon dioxide C sent to thecondenser 23 is suitably decompressed in a per se conventional way forexample by means of a valve 30.

In the decomposer 22 of the low pressure urea recovery section 21, theurea aqueous solution coming from the dissociator 17 of the mediumpressure treatment section 16 is subjected to decomposition, obtaining aconcentrated urea solution U and a second vapour phase comprisingammonia, carbon dioxide and water.

Advantageously, the concentrated urea solution U, for example with aurea concentration comprised between 60 and 80 wt. %, exits thedecomposer 22 of the urea recovery section 21 and it is used at least inpart as reactant in the melamine plant 40.

As an alternative, urea is fed into the melamine synthesis section 41 inform of molten urea coming from the final urea treatment section (notshown) provided downstream the urea recovery section 16.

To this aim, the concentrated urea solution, or a portion thereof, isfed, through flow line 34, to the melamine synthesis section 41. In theexample of FIG. 1, not all the urea produced in the plant 10 is used forthe melamine synthesis and thus a portion of the concentrated ureasolution is subjected to the final urea treatment steps (per seconventional and then not shown) of the process for urea production,such as the steps of decomposition under vacuum and granulation orprilling of the molten urea so obtained.

To control the melamine synthesis, section 41 can optionally also be fedwith an additional flow of ammonia, indicated in FIG. 1 by flow line 42.

From the melamine synthesis section 41, a melamine solution isdischarged, through flow line 43, for further processing such as cooling(not shown), where melamine is converted into a powder and exits theintegrated plant 1.

Instead the off-gases consisting essentially of carbon dioxide andammonia that are also obtained in section 41, as by-products of themelamine synthesis, leaves said section 41 through flow line 32.Generally, before leaving the melamine synthesis section 41, theoff-gases are suitably washed (scrubbed), not shown, with the feedconcentrated urea solution in order to remove possible liquid melamineentrained in such gases.

In accordance with another aspect of the integrated process according tothe present invention, the second vapour phase comprising ammonia,carbon dioxide and water obtained in the decomposer 22 of the urearecovery section 21 is sent to the condenser 23 of the same section 21and advantageously subjected to condensation, obtaining a recyclecarbamate aqueous solution.

Preferably, as represented in the example of FIG. 1, the second vapourphase comprising ammonia, carbon dioxide and water is subjected tocondensation together with the feed carbon dioxide C fed to saidcondenser 23.

A suitable amount of a carbamate aqueous solution W (carbonate) having acondensation water content comprised between 30 and 80 wt. % is also fedto the condenser 23 of the low pressure urea recovery section 21, toallow the second vapour phase and the feed carbon dioxide C,respectively, to condense to ammonium carbamate.

The carbamate aqueous solution W (carbonate) generally comes from atreatment section of the process condensate and/or from an ammonialiquor reservoir, per se conventional and not represented in FIG. 1.

In accordance with the present invention, as represented in the exampleof FIG. 1, the recycle carbamate aqueous solution obtained in thecondenser 23 of the low pressure urea recovery section 21 is, accordingto the present process, fed to the condenser 18 a of the medium pressuretreatment section 16 for the condensation of the vapour phase comprisingammonia, carbon dioxide and water coming from the medium pressuredissociator 17 as well as of the off-gases discharged from the melaminesynthesis section 41 of the melamine plant 40.

To this purpose, said off-gases are fed to the condenser 18 a of themedium-pressure section 16 through said flow line 32 while said vapourphase coming from the dissociator 17 and said recycle carbamate solutionare fed to said condenser 18 through flow lines 31 and 33 respectively.

Preferably, the condenser 18 a of the medium-pressure section 16 isoperated substantially at the same pressure as that of the off-gasesdischarged form the melamine synthesis section 41.

In the case the pressure of the off-gases coming from the melaminesynthesis section 41 is higher than the pressure of the recyclecarbamate aqueous solution discharged from the urea recovery section 16,the latter solution, as shown in FIG. 1, is advantageously compressed,through a first compression section 24, to the pressure of suchoff-gases i.e. to the operating pressure of the condenser 18 of themedium-pressure treatment section 16.

According to an alternative embodiment of the process of the presentinvention, not shown, the condensation step in the condenser 18 a of themedium-pressure treatment section 16 is of the double-effect type, inwhich the condensation heat, instead of being dissipated in a coolingfluid (generally cooling water), is advantageously exploited for furtherconcentrating the concentrated urea solution U exiting the decomposer 22of the low-pressure urea recovery section.

In this case, the condensation heat produced during condensation of thevapour phase is transmitted to the concentrated urea solution byindirect thermal exchange, allowing the decomposition and then theseparation of the part of the ammonium carbamate, ammonia and waterstill present in said solution and then further concentrating ureacontained into it.

In the example of FIG. 1, the concentrated carbamate aqueous solutionexiting the medium-pressure condenser 18 a is duly compressed in asecond compression section 19, in a per se convantial way, and isrecycled to the reactor R of the high-pressure urea synthesis section 11through the scrubber 14 and the high-pressure condenser 12.

According to an alternative embodiment of the present invention, notshown, at least of a part of the carbamate aqueous solution exiting themedium-pressure condenser 18 a is fed directly, duly compressed, to thehigh-pressure condenser 12 and it then flows to the reactor R.

The remaining part of aqueous solution comprising urea, ammoniumcarbamate and ammonia, exiting the reactor R and not sent to the mediumpressure treatment section 16, is subjected to the recovery phase of theammonium carbamate and of the ammonia present in such a solution, in thehigh pressure loop of the present process.

In particular, the remaining part of the aqueous solution comprisingurea, ammonium carbamate and ammonia exiting the reactor R of thesynthesis section 11 is fed to the high pressure stripper 13 where it issubjected to decomposition and stripping with feed carbon dioxide C. Theammonia and carbon dioxide thus produced are then recondensed(partially) into ammonium carbamate in the high pressure condenser 12and recycled in the form of ammonium carbamate to the reactor R of theurea synthesis section 11.

The condensation in the high pressure condenser 12 of the ammonia andcarbon dioxide coming from the stripper 13 is made to occur byabsorption of such gases with the feed ammonia N (liquid) and with thecarbamate aqueous solution coming, suitably compressed, from thecondenser 18 of the medium pressure treatment section 16, through thescrubber 14.

The aqueous solution comprising urea, ammonium carbamate and ammoniaobtained in the stripper 13, following the aforementioned decompositionand stripping steps with CO₂, is suitably decompressed in a per seconventional way for example by means of a valve 25 at the operatingpressure of the urea recovery section 21 and fed to the low pressuredecomposer 22 of such a section 21. Here, such a solution is subjectedto decomposition, together with said urea aqueous solution coming fromthe dissociator 17 of the medium pressure treatment section 16,obtaining the concentrated urea solution U and the second vapour phasecomprising ammonia, carbon dioxide and water, described above.

The unreacted carbon dioxide and ammonia and water in vapour phasepresent in the urea synthesis section 11, or rather in the reactor R,are made to exit the latter and fed to the high pressure scrubber 14.These vapours generally also comprise inert gases (for example air)present in the feed carbon dioxide C.

In the scrubber 14, the aforementioned vapours are subjected to awashing treatment with the carbamate aqueous solution coming, suitablycompressed, from the condenser 18 a of the medium pressure treatmentsection 16, for the recovery of the carbon dioxide and ammonia presentin them and the separation of the inert gases. The inert gases thusseparated are then released into the atmosphere in a per se conventionalmanner, moreover providing suitable decompression thereof for example bymeans of a valve 26. Alternatively, such inert gases can be recycled inother parts of the plant 10 (not represented). The carbon dioxide andammonia absorbed in the carbamate aqueous solution coming from thecondenser 18 are, on the other hand, recycled to the urea synthesissection 11, or rather to the reactor R, through the high pressurecondenser 12.

In FIG. 2, with number 110 is globally and schematically indicated anintegrated plant for urea and melamine production according to anotherembodiment of the present invention.

In the integrated plant 110, elements which are structurally orfunctionally equivalent to corresponding elements of the integratedplant 1 described above will be given the same reference numerals and,for sake of brevity, will not be further described.

The integrated plant 110 differs from the plant 1 described above inthat the condensation unit 18 of the medium-pressure treatment section16 of the urea plant 10 comprises a first condenser 18 b and secondcondenser 50 in fluid communication to each other, instead of a singlecondenser.

In addition, in the first condenser 18 b of the medium-pressuretreatment section 16 are fed the off-gases coming from the synthesissection 41 of the melamine plant, through the flow line 32 and therecycle carbamate solution coming from the condenser 23 of the urearecovery section 21, through the flow line 33, while the vapour phasecoming from the dissociator 17 is fed, through the flow line 31, to thesecond condenser 50 of the medium-pressure treatment section 16.

In the second condenser 50, said vapour phase is condensed through saidconcentrated carbamate aqueous solution obtained from the condensationof the off-gases in the first condenser 18 b, which concentratedcarbamate solution is fed to the second condenser through the flow line51.

A more concentrated carbamate solution is then obtained at the exit ofthe second condenser 50 which is recycled to the urea synthesis section11 in the way indicated above with reference to the integrated plant 1.

Preferably, in the integrated plant 110, the condensation step in thecondenser 18 b of the medium-pressure treatment section 16 is of thedouble-effect type, in which the condensation heat, instead of beingdissipated in a cooling fluid (generally cooling water), isadvantageously exploited for further concentrating the concentrated ureasolution U exiting the decomposer 22 of the low-pressure urea recoverysection.

In the integrated plants described above, the flow lines indicated inFIGS. 1 and 2 with references 27-29, 31-34, 39, 42, 43 and 51 representschematically per se conventional conduits and/or ducts.

With the process according to the invention, particularly advantageousresults have been obtained by feeding an amount of feed carbon dioxide Ccomprised between 1 and 5 wt. %, even more preferably comprised between2 and 3 wt. %, of all of the feed carbon dioxide C fed to the plant 10,to the condenser 23 of the low pressure urea recovery section 21.

Moreover, the part of aqueous solution comprising urea, ammoniumcarbamate and ammonia sent to the medium pressure treatment section 16is preferably comprised between 10 and 50 wt. %, even more preferablycomprised between 10 and 25 wt. %, of the aqueous solution coming fromthe urea synthesis section 11.

With reference to FIGS. 1 and 2, the structural features of the plants 1and 110 for the integrated urea and melamine production according to theprocess of the present invention just described shall now be betterspecified.

In accordance with the present invention, the integrated plants 1 and110 comprise each a urea plant 10 comprising a high pressure ureasynthesis section 11, a medium-pressure treatment section 16 for a partof the urea solution produced in said synthesis section 11, comprising adissociator 17 and a condensation unit 18, and a low pressure urearecovery section 21 comprising a decomposer 22 and a condenser 23, saidsections 11, 16 and 21 being in fluid communication to each other, and amelamine plant 40 comprising a melamine synthesis section 41 whereinoff-gases resulting as by-products of the melamine synthesis aredischarged from said plant 40 at a pressure of at least 2 bar andrecycled to said high pressure urea synthesis section.

The plants 1 and 110 further comprise:

-   -   connecting means 32 between said melamine synthesis section 41        and said condensation section 18 of the medium-pressure        treatment section for feeding said off-gases coming from said        melamine synthesis section 41 to said condensation section 18 of        the medium-pressure treatment section 16;    -   connecting means 33 between said condenser 23 of the urea        recovery section 21 and said condensation section 18 of the        medium-pressure treatment section 16 for feeding a recycle        ammonium carbamate coming from said condenser 23 of the urea        recovery section 21 to said condensation section 18 of the        medium-pressure treatment section 16; and    -   connecting means 29 between said dissociator 17 of the        medium-pressure treatment section and said decomposer 22 of the        low-pressure urea recovery section 21 for feeding a urea aqueous        solution obtained from dissociation in said treatment section 16        to said decomposer 22 of the urea recovery section.

According to an embodiment of the invention, said condensation sectionof the medium-pressure treatment section comprises a single condenser.

The plants 1 and 110 further comprise connecting means 31 between saiddissociator 17 and said condensation unit 18 of the medium-pressuretreatment section 16 for feeding a vapour phase comprising ammonia,carbon dioxide and water from said dissociator 17 to said condenser 18of the medium-pressure treatment section.

In the plants 1 and 110, said connecting means essentially consists ofper se conventional conduits and/or ducts.

The plants 1 and 110 further comprise a first compressing section 24located, in fluid communication, between the condenser 23 of the urearecovery section 21 and the condensation unit 18 of the medium-pressuretreatment section 16 for compressing a recycle carbamate solution comingfrom said condenser 23 of the urea recovery section 21 to a pressurecorresponding to the operative pressure of said condensation unit 18 ofthe medium-pressure treatment section 16, as well as a secondcompressing section 19 located, in fluid communication, between saidcondensation unit 18 of the medium-pressure treatment section 16 andsaid high-pressure urea synthesis section 11 for compressing aconcentrated carbamate solution coming from said condensation unit 18 ofthe medium-pressure treatment section 16 to a pressure corresponding tothe operative pressure of said high-pressure urea synthesis section 11.

In the plants 1 and 110, said first compressing section 24 and saidsecond compression section 19 comprise at least a per se conventionalpump.

According to an embodiment of the invention (integrated plant 1 of FIG.1), the condensation unit 18 of the medium-pressure treatment section 16comprises a single condenser 18 a.

According to another embodiment of the invention (integrated plant 110of FIG. 2) the condensation unit 18 of the medium-pressure treatmentsection 16 comprises a first condenser 18 b and a second condenser 50.In addition, the integrated plant 110 comprises:

-   -   connecting means 32 between said melamine synthesis section 41        and said first condenser 18 b of the medium-pressure treatment        section 16 for feeding said off-gases coming from said melamine        synthesis section 41 to said first condenser 18 b of the        medium-pressure treatment section 16;    -   connecting means 33 between said condenser 23 of the urea        recovery section 21 and said first condenser 18 b of the        medium-pressure treatment section for feeding a recycle ammonium        carbamate aqueous solution coming from said condenser 23 of the        urea recovery section 21 to said first condenser 18 b of the        medium-pressure treatment section 16;    -   connecting means 51 between said first condenser 18 b and said        second condenser 50 of the medium-pressure treatment section 16        for feeding a carbamate aqueous solution coming from said first        condenser 18 b to said second condenser 50; and    -   connecting means between said dissociator 17 and said second        condenser 50 of the medium-pressure treatment section 16 for        feeding a vapour phase comprising ammonia, carbon dioxide and        water from said dissociator 17 to said second condenser 50 of        the medium-pressure treatment section.

Furthermore, according to the invention, the integrated plants 1 and 110comprise connecting means 39 between said single condenser 18 or saidsecond condenser 50 of the medium-pressure treatment section 16 and saidurea synthesis section 11 for feeding a carbamate solution coming fromsaid single condenser 18 or said second condenser 50 in said ureasynthesis section 11, as well as connecting means 27 for feeding feedcarbon dioxide C to said condenser 23 of the low-pressure urea recoverysection 21.

Preferably, in the plant 1 of FIG. 1, the single condenser 18 a of themedium-pressure treatment section 16 comprises a conventional tubebundle, in fluid communication, on the tube side, with the concentratedurea solution U exiting the low pressure decomposer 22 of thelow-pressure urea recovery section 21 and, on the shell side, with thevapour phase comprising ammonia, carbon dioxide and water coming fromthe dissociator 17 of the medium-pressure treatment section 16, withoff-gases coming from the melamine synthesis section 41 as well as withthe recycle carbamate aqueous solution coming from the condenser 23 ofthe low-pressure urea recovery section 21.

Preferably, in the plant 110 of FIG. 2, the first condenser 18 b of themedium-pressure treatment section 16 comprises a conventional tubebundle, in fluid communication, on the tube side, with the concentratedurea solution U exiting the low pressure decomposer 22 of thelow-pressure urea recovery section 21 and, on the shell side, withoff-gases coming from the melamine synthesis section 41 as well as withthe recycle carbamate aqueous solution coming from the condenser 23 ofthe low-pressure urea recovery section 21.

From the previous description it can clearly be seen that the integratedprocess for urea and melamine production according to the inventionsolves the technical problem and achieves numerous advantages the firstof which lies in the fact that a high overall conversion yield isobtained in the high pressure loop, and in particular in the ureasynthesis section, for example comprised between 58 and 62 wt. %, evenin the case of revamping pre-existing high-capacity integrated plants inwhich for example the urea plant can produce between 3000 and 4500Metric Ton/day of urea.

A further advantage lies in that the more concentrated carbamateaquesous solution can be recycled to the synthesis section of the ureaplant, obtaining advantageously a higher urea conversion yield and areduction of energy consumption since the condensation of off-gases isperformed by exploiting the low amount of water already contained in therecycle carbamate aqueous solution obtained in the urea recovery sectionof the urea plant which is anyway recycled to the high-pressuresynthesis section of the urea plant.

A further advantage is that, thanks to the present invention and inparticular to the high conversion yield, it is possible to reduce theenergy consumption of the high pressure synthesis loop as well as of thelow pressure urea recovery section, with respect to the processesaccording to the prior art. It follows from this that with the sameenergy consumption and size of the apparatuses that constitute the plantfor urea production, the process according to the present inventionallows operation in such a plant with a higher production capacity withrespect to what is allowed with the processes according to the priorart. In other words, with the same production capacity, the plantintended to carry out the process according to the present invention issmaller in size, and thus more cost-effective and with less operatingcosts, with respect to the plant necessary to obtain such a capacitywith the methods of the prior art.

Moreover, the actuation of the process is particularly simple andreliable, and does not require large investment costs.

Among the numerous advantages achieved by the present invention, it isworth to cite also the possibility of revamping pre-existing plants forthe urea and melamine integrated production, for the purpose ofincreasing the production capacity and the production yield of the ureaplant as well as of reducing the relative energy consumption with theprocess according to the invention.

Of course, a man skilled in the art may contribute numerousmodifications and changes to the process for the urea and melamineintegrated production described above for satisfying specific andcontingent requirements, all falling within the scope of protection ofthe invention itself, as it is defined by the following claims.

1. An integrated process for urea and melamine production, wherein ureais produced in a urea plant comprising a high-pressure urea synthesissection from which a aqueous solution comprising urea, ammoniumcarbamate and ammonia is obtained, and a low-pressure urea recoverysection, and melamine is produced in a melamine plant wherein off-gasesresulting as by-products of the melamine synthesis are dischargedtherefrom at a medium pressure and recycled to said high pressure ureasynthesis section, comprising the steps of: feeding at least a part ofsaid aqueous solution comprising urea, ammonium carbamate and ammoniacoming from said urea synthesis section to a medium-pressure treatmentsection of the urea plant for recovering ammonium carbamate and ammoniacontained in it; subjecting said part of aqueous solution comprisingurea, ammonium carbamate and ammonia to dissociation in saidmedium-pressure treatment section, obtaining a urea aqueous solution anda vapour phase comprising ammonia, carbon dioxide and water; feedingsaid urea aqueous solution obtained from dissociation in said treatmentsection to a decomposer of a urea recovery section operating at apredetermined low pressure, subjecting said urea an solution todecomposition in said decomposer of said urea recovery section,obtaining a concentrated urea solution and a second vapour phasecomprising ammonia, carbon dioxide and water; subjecting said secondvapour phase to condensation in a condenser of said urea recoverysection in fluid communication with said decomposer, obtaining a recycleammonium carbamate aqueous solution; feeding said off-gases coming fromsaid melamine plant and said recycle ammonium carbamate solution to acondensation section of said medium-pressure treatment section of theurea plant; condensing said offs-gases with said recycle carbamateaqueous solution in said condensation unit of the medium-pressuretreatment section, obtaining a concentrated carbamate aqueous solution;and recycling said carbamate aqueous solution to said high pressure ureasynthesis section.
 2. The process according to claim 1, wherein saidmedium-pressure treatment section of the urea plant operates to pressuresubstantially equal or lower than that of said off-gases.
 3. The processaccording to claim 1, wherein said recycle carbamate solution comingfrom the urea recovery section is fed directly to said medium-pressuretreatment section for condensing said off-gases.
 4. The processaccording to claim 1, wherein said recycle carbamate solution comingfrom the urea recovery section is compressed to a pressure substantiallycorresponding to the operating pressure of said medium-pressuretreatment section for condensing said off-gases, before feeding it intosaid condensation unit.
 5. The process according to claim 1, whereinsaid concentrated carbamate aqueous solution coming from saidcondensation unit of the medium-pressure treatment section is compressedto a pressure substantially corresponding to the operating pressure ofsaid urea synthesis section, before feeding it to said urea synthesissection.
 6. The process according to claim 1, wherein said condensationunit of the medium-pressure treatment section comprises a singlecondenser and in that the process further comprises the steps of:feeding said vapour phase comprising ammonia, carbon dioxide and water,said off-gases and said recycle ammonium carbamate solution in saidsingle condenser of the medium-pressure treatment section; condensingsaid vapour phase comprising ammonia, carbon dioxide and water as wellas said off-gases with said recycle ammonium carbamate solution in saidsingle condenser of said medium-pressure treatment section, obtaining aconcentrated ammonium carbamate aqueous solution; and recycling saidconcentrated ammonium carbamate aqueous solution to said high pressureurea synthesis section,
 7. The process according to claim 1, whereinsaid condensation unit of the medium-pressure treatment sectioncomprises a first condenser and a second condenser in fluidcommunication to each other and in that the process further comprisesthe steps of: feeding said off-gases coming from said melamine plant andsaid recycle ammonium carbamate solution to said first condenser of themedium-pressure treatment section of the urea plant; condensing saidoff-gases with said recycle carbamate aqueous solution in said firstcondenser of the medium-pressure treatment section, obtaining a firstconcentrated ammonium carbamate aqueous solution; feeding said firstconcentrated ammonium carbamate aqueous solution in said secondcondenser of the medium-pressure treatment section; feeding said vapourphase comprising ammonia, carbon dioxide and water obtained fromdissociation of said part of the aqueous solution comprising urea,ammonium carbamate and ammonia, in said second condenser of themedium-pressure treatment section; and condensing said vapour phasecomprising ammonia, carbon dioxide and water with said firstconcentrated ammonium carbamate aqueous solution in said secondcondenser of the medium-pressure treatment section, obtaining a secondconcentrated ammonium carbamate aqueous solution; and recycling saidsecond concentrated ammonium carbamate aqueous solution to said highpressure urea synthesis section.
 8. The process according to claim 1,wherein it comprises the further steps of: feeding carbon dioxide tosaid condenser of said urea recovery section; and subjecting said carbondioxide and said second vapour phase to condensation in said condenserof said urea recovery section, obtaining a recycle ammonium culminateaqueous solution.
 9. The process according to claim 8, characterized byfeeding a carbon dioxide amount from 1 to 10 wt. % of the totality offeed carbon dioxide to said condenser of said urea recovery section. 10.The process according to claim 1, wherein said part of aqueous solutioncomprising urea, ammonium carbamate and ammonia fed to said treatmentsection operating at medium pressure is comprised between 10 and 50 wt.% of said aqueous solution comprising urea, ammonium carbamate andammonia obtained in said synthesis section.
 11. The process according toclaim 1, wherein said medium pressure of the treatment section iscomprised between 10 and 70 bar.
 12. An integrated plant for urea andmelamine production, wherein urea is produced in a urea plant comprisinga high-pressure urea synthesis section and a low-pressure urea recoverysection comprising a decomposer and a condenser, said sections being influid communication to each other, and melamine is produced in amelamine plant wherein off-gases resulting as by-products of themelamine synthesis are discharged from said plant at a medium pressureand recycled to said high-pressure urea synthesis section, the plantcomprising: a medium-pressure treatment section of the urea plant of apart of the urea solution produced in said synthesis section, comprisinga dissociator and a condensation unit; connecting means between saidmelamine synthesis section and said condensation unit of themedium-pressure treatment section for feeding said off-gases coming fromsaid melamine synthesis section to said condensation unit of themedium-pressure treatment section; connecting means between saidcondenser of the urea recovery section and said condensation unit of themedium-pressure treatment section for feeding a recycle ammoniumcarbamate coming from said condenser of the urea recovery section tosaid condensation unit of the medium-pressure treatment section; andconnecting means between said dissociator of the medium-pressuretreatment section and said decomposer of the low-pressure urea recoverysection for feeding a urea aqueous solution obtained from dissociationin said treatment section to said decomposer of the urea recoverysection.
 13. The integrated plant according to claim 12, wherein itfurther comprises connecting means between said dissociator and saidcondensation unit of the medium-pressure treatment section for feeding avapour phase comprising ammonia, carbon dioxide and water from saiddissociator to said condenser of the medium-pressure treatment section.14. The integrated plant according to claim 12, comprising a firstcompressing section located, in fluid communication, between thecondenser of the urea recovery section and the condensation unit of themedium-pressure treatment section for compressing a recycle carbamatesolution coming from said condenser of the urea recovery section to apressure corresponding to the operative pressure of said condensationunit of the medium-pressure treatment section.
 15. The integrated plantaccording to claim 12, wherein it further comprises a second compressingsection located, in fluid communication, between said condensation unitof the medium-pressure treatment section and said high-pressure ureasynthesis section for compressing a concentrated carbamate solutioncoming from said condensation unit of the medium-pressure treatmentsection to a pressure corresponding to the operative pressure of saidhigh-pressure urea synthesis section.
 16. The integrated plant accordingto claim 12, wherein it further comprises connecting means for feedingfeed carbon dioxide to said condenser of the low-pressure area recoverysection.
 17. The integrated plant according to claim 12, characterizedin that said condensation unit of the medium-pressure treatment sectioncomprises a single condenser.
 18. The integrated plant according toclaim 17, wherein said single condenser of the medium-pressure treatmentsection comprises a conventional tube bundle, in fluid communication, onthe tube side, with the concentrated urea solution exiting thedecomposer of the low-pressure urea recovery section and, on the shellside, with the vapour phase comprising ammonia, carbon dioxide and watercoming from the dissociator of the medium-pressure treatment section,with off-gases coming from the melamine synthesis section as well aswith the recycle carbamate aqueous solution coming from the condenser ofthe low-pressure urea recovery section.
 19. The integrated plantaccording to claim 12, wherein said condensation unit of themedium-pressure treatment section comprises a first condenser and asecond condenser and in that it further comprises: connecting meansbetween said melamine synthesis section and said first condenser of themedium-pressure treatment section for feeding said off-gases coming fromsaid melamine synthesis section to said first condenser of themedium-pressure treatment section; connecting means between saidcondenser of the urea recovery section and said first condenser of themedium-pressure treatment section for feeding a recycle ammoniumcarbamate aqueous solution coming from said condenser of the urearecovery section to said first condenser of the medium-pressuretreatment section; connecting means between said first condenser andsaid second condenser of the medium-pressure treatment section forfeeding a carbamate aqueous solution coming from said first condenser tosaid second condenser; and connecting means between said dissociator andsaid second condenser of the medium-pressure treatment section forfeeding a vapour phase comprising ammonia, carbon dioxide and water fromsaid dissociator to said second condenser of the medium-pressuretreatment section.
 20. The integrated plant according to claim 19,wherein said first condenser of the medium-pressure treatment sectioncomprises a conventional tube bundle, in fluid communication, on thetube side, with the concentrated urea solution exiting the decomposer ofthe low-pressure urea recovery section and, on the shell side, withoff-gases coming from the melamine synthesis section as well as with therecycle carbamate aqueous solution coming from the condenser of thelow-pressure urea recovery section.
 21. The integrated plant accordingto claim 12, comprising connecting means between said single condenseror said second condenser of the medium-pressure treatment section andsaid urea synthesis section for feeding a carbamate solution coming fromsaid single condenser or said second condenser in said urea synthesissection.